This application claims the priority of Swiss patent application 806/00, filed Apr. 25, 2001, the disclosure of which is incorporated herein by reference in its entirety.
The invention relates to a flow sensor for liquids and a method for measuring the flow amount of a liquid.
A flow sensors for liquids is described in the following publication: M. Ashauer, H. Glosch, F. Hedrich, N. Hey, H. Sandmaier und W. Lang, xe2x80x9cThermal Flow Sensor for Liquids and Gasesxe2x80x9d, IEEE Proceedings (98CH36176) des 11. internationalen Workshops xe2x80x9cMicro Electro Mechanical Systemsxe2x80x9d, 1998.
Flow sensors for liquids serve to measure the flow velocity of a liquid. In particular, there are thermal flow sensors, which comprise a heating element in thermal contact with the liquid and a temperature sensor. Since the flow velocity affects the thermal drain from the temperature sensor as well as the thermal distribution in the liquid, the signal measured by the temperature sensor depends on the flow velocity.
From the measuring results of flow sensors for liquids, it is possible to determine the flow amount of the liquid, i.e. the amount of liquid passing a measuring point.
Hence, it is a general object of the invention to provide a sensor of the type mentioned initially that can be manufactured easily and that is still accurate. It is also an object to provide a method of the type mentioned initially that allows a very accurate determination of the flow amount.
This object is achieved the flow sensor and the method according to the invention.
The most important components of the flow sensor, i.e. the temperature sensor and the heating element, are therefore arranged on a common semiconductor substrate together with a processing and control circuit. This makes manufacturing cheaper and reduces the susceptibility to noise.
Preferably, the flow sensor is provided with a cover layer for protecting the temperature sensor and the heating element. A cover layer of DLC (Diamond-Like Carbon) has been found to be especially suited because the same is characterized by high hardness and stability.
In order to calibrate the flow sensor, it is preferably provided with a valve, which is closed by the processing and control circuit for determining the parameters of the sensor under defined conditions.
In the method according to the invention, the liquid is led through a duct past a heating element. A measuring parameter is determined that depends on the power dissipated by the heating element or from a temperature in the duct at the heating element. This allows to detect the presence of gas bubbles, which makes it possible to account for the gas bubble in the flow amount and/or to issue a warning. By means of this a more accurate measuring result is achieved.
Preferably, upon presence of a gas bubble, the measurement of the flow amount, which corresponds to an integration of the flow velocity over time, is interrupted. It is also possible to determine the volume of the gas bubble from the current flow velocity of the liquid and a temporal length of the gas bubble. Hereby, xe2x80x9ctemporal lengthxe2x80x9d is understood to be the time during which the presence of the gas bubble is detected by means of the measuring parameter.
If temperature sensors are provided before and after the heating element, not only the flow velocity of the liquid can be determined from their temperature difference, but also the flow velocity of the gas in a gas bubble. As the signals during a gas bubble are, however, substantially weaker, it is in this case not necessary to interrupt the integration for the flow amount during the presence of a gas bubble.
The device according to the invention and the method according to the invention are especially suited for determining small flow amounts, such as they have e.g. to be measured when administering medication or in process technology.